Concentrated solar power (CSP) is power generation performed in such a manner that sunlight is reflected by a mirror and concentrated in a receiver, then electric power is generated by the heat of the sunlight. CSP is roughly classified into a trough type and a tower type. The trough type is performed by concentrating sunlight onto a linear receiver by a cylindrical paraboloidal type mirror, meanwhile the tower type is performed by concentrating sunlight onto a receiver having a dot shape (several meters square) arranged on a tower, by a mirror, called heliostat, dispersed on the ground. At present, the biggest problem with these types of CSP is that both of the types have a low conversion efficiency, namely, only approximately 15%.
The biggest factor in determining the conversion efficiency is a receiver as a heat-absorbing material. The desired characteristics for receivers are such that a receiver efficiently absorbs the concentrated sunlight and the receiver itself hardly lets heat escape.
In CSP, the temperature of a receiver reaches a high temperature, namely, 400 degrees C. or more, and accordingly heat radiation loss due to this cannot be ignored. The intensity and wavelength distribution of heat radiation is expressed by a function of the temperature of an object according to Planck's law.
FIG. 37 illustrates graphs showing spectra of solar radiation and heat radiation. The figure shows heat radiation at a temperature of 200 degrees C., 400 degrees C., and 800 degrees C., and at the higher temperature, the distribution thereof moves to the shorter wavelength side. The wavelength region of heat radiation at approximately 6000 degrees C. naturally coincides with the wavelength region of solar radiation.
As shown in FIG. 37, when the spectra of solar radiation and heat radiation are compared, it is understood that solar radiation is stronger in the visible region, meanwhile heat radiation is stronger in the infrared region. Hence, as a receiver, there is useful a wavelength-selective material having higher absorption in the visible region and lower radiation in the infrared region, in other words, a wavelength-selective material having higher absorption in the visible region and lower absorption in the infrared region.
Receivers having wavelength selectivity have been already developed and some of the receivers have been commercialized. For example, Non-patent Literature 1 takes a general view of receivers having wavelength selectivity, and describes the classification of the principle thereof and a temperature range in which, receivers, including a material having no wavelength selectivity, can be used. Furthermore, Non-patent Literature 2 describes that a cermet (mixture of a metal and a ceramic) of molybdenum and alumina is film-formed on a metal to be a double layer, whereby, while aiming at achieving high heat resistance, the wavelength region for absorption is controlled. Furthermore, ALANOD, ACKTAR Ltd. and the like have actually commercialized absorbing materials having wavelength selectivity.